One-phase electronically commutated motors are very inexpensive, and are often used for specific driving tasks, e.g. for fans or centrifugal pumps. They are usually controlled by means of a Hall sensor which magnetically detects the instantaneous rotational position of the rotor. Commutation without a sensor, referred to using the term “sensorless,” is, however, desirable, since better efficiency is obtained as a result.
The terminology of such motors is somewhat confusing. For accurate definition of an ECM, firstly the number of stator current pulses per rotor rotation of 360° el. is indicated, e.g. single-pulse, two-pulse, three-pulse, etc.; also the number of winding strands in the stator is indicated, e.g. single-strand, two-strand, three-strand, etc.
An ECM can therefore e.g. be described as single-strand and two-pulse, or two-strand and two-pulse. The expression “collectorless motor” is also used instead of “ECM”. Because there is no difference between the single-strand and two-strand motors in terms of physical operation, and because simplified terminology is always desirable for practical use, such motors are generally referred to as “one-phase” ECMs, even though they can have either only a single strand or, alternatively, two strands.
Because the rotor in such motors has rotational positions at which the motor cannot generate any electromagnetic torque, an auxiliary torque is used that is effective at those zero positions. This can be a magnetically generated auxiliary torque, which is referred to as reluctance torque. Alternatively, this auxiliary torque could be generated mechanically, for example by means of a spring that is tensioned in certain rotational positions and delivers its stored energy at the zero positions. The result is that the rotor, at a standstill, is rotated sufficiently that at startup it is not in a rotational position in which the motor cannot generate an electromagnetic torque, since otherwise the motor would not be able to start. This starting position is also referred to as a “cogging” position.
When such motors are currentless, normally the rotor is at a standstill and is in a so-called cogging position, into which it is pulled by the aforesaid auxiliary torque. When current is applied to the motor with the rotor in this position, the rotor will move; it is, however, only possible to guess how strongly it will move.